Some say setting it out in the direct sun for a couple of hours works.

I've never had a fungused lens myself, just repeating things I have heard work.

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That may work but not for the reasons people say it does. The reasoning is that UV from sunlight will "kill" the fungus. If it does anything, it would deactivate the fungus so that it couldn't reproduce. The existing fungus would still be there. The problem is that glass is a very good UV filter. Very good. So there's not much UV getting into a lens through the glass or body. So other than killing any on the surface, it's not going to help. Leaving it in the sun may kill the fungus if you live in a very sunny area and it's very hot. It's the heat that kills it, not UV. That would also explain why some people say the fungus goes away after being in the sun for hours/days. If it was simply killed by UV, the dead fungus should still be there. If it was killed by heat, the heat would dry out the fungus causing it to shrink or even flake off as powder.

That may work but not for the reasons people say it does. The reasoning is that UV from sunlight will "kill" the fungus. If it does anything, it would deactivate the fungus so that it couldn't reproduce. The existing fungus would still be there. The problem is that glass is a very good UV filter. Very good. So there's not much UV getting into a lens through the glass or body. So other than killing any on the surface, it's not going to help. Leaving it in the sun may kill the fungus if you live in a very sunny area and it's very hot. It's the heat that kills it, not UV. That would also explain why some people say the fungus goes away after being in the sun for hours/days. If it was simply killed by UV, the dead fungus should still be there. If it was killed by heat, the heat would dry out the fungus causing it to shrink or even flake off as powder.

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I have thought according to same lines myself. There is one problem with heat. It will liquidate helicoid grease and it can run to lenses or aperture blades.

I have seen a couple of lenses where the "fungus" was fungus but in many cases it has to be something else. I suspect the helicoid grease as these lenses didn't respond to water, ammonia or isopropyl alcohol but were removed with lighter fluid, hand cream or acetone (that is with fat solvents).

Those lenses which have had fungus growing (or I think so) have had tiny threads growing from perimeter of spot and have been easy to clean (water wash with a drop of ammonia and isopropyl alcohol and 5 minutes in ultrasound cleaner, final wash with pure isopropyl alcohol).

I have seen a couple of lenses where the "fungus" was fungus but in many cases it has to be something else. I suspect the helicoid grease as these lenses didn't respond to water, ammonia or isopropyl alcohol but were removed with lighter fluid, hand cream or acetone (that is with fat solvents).

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Exactly. I found that about 75% of my fungus lenses didn't have fungus but grease. It was grease that had evaporated and condensed on the lens. The hallmark are these pretty uniformed spaced little spots everywhere. After I opened it up to clean, it was clearly oil/grease. Another 15% are so was not fungus but lens separation. This is easy to mistake for fungus. The tell is that it won't come off. It's between the lens elements, not on them. The remaining 10% was indeed fungus. I just wipe it off with some alcohol.

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